What it is, why they are needed and how they differ ????

Question

alignof and __alignof,
_asm and __asm,
await and __await

Answer 1

https://en.cppreference.com/w/cpp/language/alignof
https://en.cppreference.com/w/cpp/language/asm

as for await there's no direct definition on cppreference so here's this:

https://blogs.msdn.microsoft.com/vcblog/2013/12/20/asynchronous-programming-in-c-using-resumable-functions-and-await/